As to hydraulic control valve apparatus or hydraulic drive systems for use in construction machines such as hydraulic excavators, the following prior arts are known:
(1) JP, A, 60-5928
(2) JP, A, 62-38496
(3) JP, U, 59-51861
(4) JP, A, 60-11706
(5) JP, A, 2-134402
(6) JP, A, 58-501781
(1) JP, A, 60-5928, (2) JP, A, 62-38496 and (3) JP, U, 59-51861 describe hydraulic drive systems using valve apparatus which include flow control valves of center bypass type. The term "flow control valve of center bypass type" means a valve of the type that a center bypass passage for communicating a pump port with a reservoir is throttled depending on the amount of movement of a spool. With the center bypass passage throttled, the pump delivery pressure is raised to supply a hydraulic fluid to an actuator via a feeder passage and a meter-in variable throttle. A load check valve for preventing the hydraulic fluid from flowing reversely is installed in the feeder passage.
Also, the hydraulic drive systems described in (1) JP, A, 60-5928, (2) JP, A, 62-38496 and (3) JP, U, 59-51861 are arranged to be used with hydraulic excavators. Of them, in the hydraulic drive system of (1) JP, A, 60-5928, a swing preference switch valve operable in response to a swing pilot pressure is connected between an arm flow control valve for supplying a hydraulic fluid to an arm cylinder of the hydraulic excavator and a pump port so that, when a swing flow control valve for supplying the hydraulic fluid to a swing motor and the arm flow control valve are simultaneously driven, a flow of the hydraulic fluid supplied to the arm flow control valve is throttled to raise a pressure of the hydraulic fluid supplied to the swing flow control valve.
In the hydraulic drive system of (2) JP, A, 62-38496, for the same purpose, a variable sequence valve operable in response to the swing pilot pressure is connected between the arm flow control valve and the pump port.
In the hydraulic drive system of (3) JP, U, 59-51861, for the same purpose, the pressure at an inlet port of the swing flow control valve is introduced as a pilot pressure to a load check valve installed at an inlet port of the arm flow control valve so that the pilot pressure pushes a popper of the load check valve to move, thereby throttling a flow of the hydraulic fluid passing through the load check.
Further, (4) JP, A, 60-11706 and (5) JP, A, 2-134402 describe hydraulic drive systems using valve apparatus which include flow control valves of closed center type. The term "flow control valve of closed center type" means a valve of the type that a pump port is not communicated with a reservoir regardless of the position of a spool, and that is usually employed in combination with a load sensing system for controlling the delivery rate of a hydraulic pump depending on a load pressure. Additionally, a pressure compensating valve is installed upstream of the flow control valve of closed center type so that operating speeds of a plurality of actuators will not be changed with variations in loads. A load check valve for preventing the hydraulic fluid from flowing reversely is disposed between the pressure compensating valve and the flow control valve.
In the valve apparatus described in (5) JP, A, 2-134402, particularly, when the flow control valve of spool type, the pressure compensating valve and the load check valve are combined to construct a hydraulic control valve apparatus, these three valves are assembled in one block to provide a single valve apparatus for purposes of reducing the number of piping lines and rendering the apparatus compact.
On the other hand, (6) JP, A, 58-501781 proposes a hydraulic control valve apparatus of not spool type but seat valve type. This hydraulic control valve apparatus comprises a seat valve and a pilot control valve combined with each other.